The present invention generally relates to shock absorbing seats, and in particular vehicle seats with seat cushions that incorporate energy absorbing materials such as viscoelastic foams to minimize occupant injury during high acceleration events.
Seats designed to attenuate high impact loads are well known in various military and civilian vehicle and aircraft applications. Military vehicles in particular may in normal course be exposed to extreme impacts from mine blasts or improvised explosive devices. Variable displacement, or energy attenuating seats are commonly employed to absorb and attenuate lethal energy pulse levels that may occur in an otherwise survivable blast or crash event. One such energy attenuating system is disclosed in patent application Ser. No. 12/384,061 assigned to the assignee of the present invention. The disclosed troop seat utilizes a sacrificial metal link that attenuates an energy pulse by collapsing in a controlled fashion as the seat strokes downward relative to a vehicle compartment, substantially reducing the peak acceleration felt by the occupant. However, testing and experience have shown that injury can occur not only during the initial impact, but also during rebound as compressive energy stored in the seat structure or seat cushion is released.
Common polyurethane foam used for vehicle seat cushions has been shown to contribute to post slam-down rebound accelerations that exceed accepted injury threshold levels as expressed in DRI (Dynamic Response Index). DRI is a dimensionless number related to spinal compression. For example, NATO document AEP-55, Volume 2 (Edition 1) entitled “Procedures for Evaluating the Protection Level of Logistic and Light Armored Vehicles—Mine Threat” specifies a DRI limit of 17.7 for seated vehicle occupants. Thus while the use of an EA seat can substantially reduce the probability of injury from the initial impact, injury may nevertheless occur during the subsequent rebound.
To counteract the rebound effect, many modern vehicle seat cushions utilize viscoelastic foams, also known as memory foams. As contrasted with polyurethane foams which rebound almost immediately after being compressed, viscoelastic foams generally rebound very slowly, and are thus more effective at mitigating seat cushion induced rebound acceleration. However seat cushions made of viscoelastic foams can be uncomfortable for prolonged use. Under the weight of a sitting occupant, viscoelastic foams tend to compress to the point of becoming substantially inelastic, and can feel quite hard. Also, because of the viscous nature of the foam and its tendency to conform, seat cushions made of viscoelastic foams are generally unable to retain their initial shape. Consequently any beneficial pressure distribution attributed to a particular sculpted shape is largely lost once the cushion fully compresses and conforms under the weight of an occupant.